This invention relates generally to voice privacy systems and, more particularly, to a voice privacy system adapted to enhance the privacy of a transmission by disguising the amplitude characteristics of transmitted encoded signals.
Privacy systems are well known for rendering audio signals, particularly voice signals, unintelligible for transmission over an exposed transmission link so as to maintain the transmission private and to avoid reconstruction of the signal content by unauthorized listeners. In such systems, the voice signals are typically encoded at a transmitting site using an encoding technique that involves scrambling or displacing the signals in the frequency domain, time domain or both. At receiving site, the scrambled signals are decoded by, in effect, reversing the encoding procedure to recover the original signals. Ideally, in any system of this type, the encoding technique used should make it extremely difficult for unauthorized listeners to decode or "break" an intercepted scrambled signal, yet still permit recovery at the receiving site of the transmitted information with good intelligibility and recognition by authorized listeners.
Most encoding techniques presently in use, whether involving frequency scrambling, time scrambling or both, have difficulty disguising amplitude variations in the transmitted signals. This can be particularly problematic with voice signals since the amplitude of the typical voice signal varies in a more or less regular manner that is related to the cadence and intersyllabic rate of speech and the presence of certain well defined, recurring phonemes. A simple illustration is a number count of one to ten. Under most conditions, a listener, by analyzing the cadence of a scrambled signal, can determine that a series of numbers is being counted, even though the frequency content of the signal has been severely disturbed. Information that an unauthorized listener is able to extract from a scrambled signal concerning its cadence, pauses and interruptions can serve as a starting point for further breakdown of the signal. Thus, to provide enhanced levels of privacy and security, it is desirable to utilize encoding techniques that effectively mask or disguise the amplitude characteristics of the transmitted signals.
A variety of attempts have heretofore been made for disguising amplitude characteristics in privacy systems. One of the most straightforward techniques that has been used for this purpose involves severely limiting the amplitude of the signals prior to their transmission. This technique, however, is only partially effective at best, as it does not hide complete pauses in the signals. Amplitude limitation of the transmitted signals also generally makes it more difficult to detect and completely recover the original signals at the receiving side of the system.
Time division privacy systems of the type described, for example, in U.S. Pat. No. 3,824,467, also provide some degree of amplitude masking. In systems of this type, the voice signals are first divided into small time increments and the time increments are then rearranged to form an unintelligible transmitted signal. Time division scrambling systems, however, generally require components that temporarily store the various signal increments and components that selectively control the storage components to effect the rearrangement of the increments. These components must be present at both the transmitting and receiving side of the system and add significantly to the complexity and cost of the system. Additionally, time division privacy systems are still only partially effective in disguising amplitude variations in the signals, for complete pauses that fall within individual time increments still remain as pauses in the transmitted signals.
Another approach to the amplitude masking problem is that described in U.S. Pat. No. 3,978,288. In accordance with the technique described in that patent, filling signals having characteristics corresponding to those of the voice signals to be transmitted are selectively inserted into the pauses normally encountered in the voice signals as a means of masking or disguising the pauses. The insertion of the filling signals may take place either before or after the signals are encoded. As can be appreciated from a review of U.S. Pat. No. 3,978,288, the apparatus necessary to implement the filling signal insertion technique is rather complex and expensive. The filling signal insertion technique is also disadvantaged by the fact that a decoding signal must be transmitted with the encoded voice signal but in a separate channel to enable the removal or suppression of the filling signals at the receiving site. Additionally, the filling signal insertion technique is only effective in disguising complete pauses in the transmitted signals. Amplitude variations in the non-zero, speech portions of the transmitted signals are not affected by the filling signals.